Hydraulic systems which lift heavy loads by spool type directional control valves have utilized an additional valve in the circuit commonly known in the trade as a "lock-out" valve which is positioned in the lines between the directional control valve and the working cylinder. The need for these additional lockout valves is caused by normal spool leakage in a directional control valve which if left in neutral would slowly allow the load to drop due to normal tolerance leakage across the spool lands of the control valve.
A lockout valve is a spring-closed check valve positioned in each motor port of the control valve which prevents any leakage from the cylinder to the valve until they are forced open by pressure from the pump source. The prior art lockout type control valves, such as shown in U.S. Pat. No. 3,596,566, depend on pressure fluid flowing from the pump source outward through one or the other motor ports of the valve to open both lockout check valves. In such a system when the piston rod of the controlled motor is subjected to a large external load, there is a great pressure variation in the fluid flow path in which each lockout check valve is interposed. As the piston rod is accelerated due to the external load, the cylinder begins to outrun the pump causing a drop in pressure in the pump supply path to one side of the cylinder. This lowers the pressure tending to hold the lockouts open and both lockouts momentarily spring-close. Their closing causes an instant stoppage of the cylinder piston and a resultant shock to the entire system. When fluid from the pump source again builds up sufficient pressure, both lockouts are reopened and the cycle is repeated. This phenomenon is known in the valve art as lockout chatter and regardless of efforts, the valve operator is incapable of accurately positioning the cylinder rod and its attached load or of accurately controlling the rate of movement of the load. All of this is due to the above-described instability of the lockout checks under heavy load.
Past efforts to overcome the lockout chatter problem, such as severely restricting the maximum return flow from the motor, have created a large operating efficiency loss which drastically limits the speed of movement and reaction time of cylinder movement on the machine.
One method of solving this chatter problem is illustrated in the above-mentioned patent wherein the timing of the control valve is changed so that the lockouts are fully open before there is any flow to or from the motor with the utilization of a separate pressure cavity to hold the lockouts open. This system requires a very complex valve coring and spool design to provide for all of the additional passages and valving functions. While the system does decrease the possibilities of chatter, it does not prevent the motor from overrunning the pump and cavitating the system.